The Structure and Properties of MoSi2-Mo5Si3 Composites

Abstract

ABSTRACTThe addition of Mo5Si3 as a reinforcing second phase in a MoSi2 matrix has been investigated for possible high temperature strengthening effects. MoSi2 with up to 45 vol % Mo5Si3 was fabricated using powder metallurgy (PM) and arc-casting (AC) techniques. Effects of processing routes, which result in different microstructures, on their mechanical properties are given. PM composites, which have an equiaxed microstructure, exhibit a limited increase in hardness. Higher hardnesses are observed in script-structured AC eutectics and Er-modifiedeutectics throughout the temperatures studied (25–1300°C). Crack propagation paths induced by indentation show long transphase cracks in the AC materials vs short intergranular and interphase cracks in the PM composites at high temperatures.Transmission electron microscopy discloses that the interface in the AC composites has a low-index orientation relationship between the two phases and shows regularly faceted interfacial structures, while planar interfaces are found in the PM composites. These observations suggest the interface is stronger and lower in energy in the AC composites, which is consistent with the higher hardness values and long transphase cracks observed.Dislocation analysis shows the presence of ordinary dislocations (<100>, <110> and 1/2<111>) in MoSi2 in the as-fabricated composites. These types of dislocation are also responsible for the high temperature plastic deformation in compression in both the monolithic MoSi2 and the composites. <331> types of dislocation are only found in MoSi2 either near the interface of the AC composites or in materials deformed below 1000°C.